The invention relates to an apparatus and method for generating position-dependent signals, and, more particularly, to an apparatus and method which allows single-field phase-opposition scanning. German Patent DE 34 16 864 C2 discloses a measuring instrument having a scale with a reference mark whose graduation markings are embodied as transverse phase gratings. In-phase and phase-opposition signals can be derived from a reference mark field.
Soviet Union Patent Disclosure SU 1292181 A1 describes a similar instrument, in which an encoded scale graduation is scanned, the graduation markings of which are embodied in the form of transverse phase gratings. The grating constants of these transverse phase gratings of individual graduation markings differ so that the incident light is deflected in different directions and detected by various photoelements. The transverse phase gratings are embodied nonperiodically, so as to focus the incident collimated light in a certain direction.
With reference to both of the measuring instruments described above, the beam originating at the light source interacts only once with the transverse phase gratings of the scale. The beam is split transversely into various partial beams that are then no longer superimposed as the beam course continues. In order to attain significant transverse splitting of these partial beams, the grating constants of the transverse phase graduations must be quite small. This is unfavorable since fine transverse phase grating must be applied to the scale, which can also be quite large, thereby making stringent demands on the production processes, and causing production costs to rise considerably.
In addition, the reference mark disclosed in DE 34 16 864 C2 is based on the principle of shadow casting which only allows brief pulses to be generated at a very short spacing distance.
European Patent Disclosure EP 0 363 620 B1 discloses a measurement instrument operating on the principle of shadow casting scanning wherein short reference pulses are generated by two slightly displaced reference mark fields. Separating the beams of the reference pulse single-phase signals from one another and from the incremental signals requires certain optical configurations which lead to large dimensions of the scanner head. Moreover, at least three photoelements and correspondingly many electrical signal lines are required for the reference mark alone.
According to European Patent Disclosure EP 0 513 427 A1, a brief reference pulse is generated at a long spacing distance with the aid of chirped graduations and interferential scanning. The scanning principle requires that the resultant orders of diffraction emerging from the graduation arrangement be distributed to various photoelements. If wider reference pulses are to be generated, then larger local graduation periods of the chirped graduations are needed, and correspondingly longer focal lengths of the lenses are needed to separate the orders of diffraction. Once again, a small structure of the scanner head is impossible.
German Patent DE 42 12 281 C2 discloses an interferential reference mark which, because of the use of an amplitude scanning graduation, eliminates the need to separate the resultant orders of diffraction. In this way, reference pulses with a width even greater than approximately 1.mu. can be generated with a small structural form. Scanning with an amplitude structure does lower the useful portion of the reference pulse so that only a simultaneous generation of in-phase and phase opposition signals from one graduation field would assure adequate insensitivity to noise or interference. This kind of single-field phase-opposition scanning, however, is not possible with the reference mark described.